General liquid developers for electrophotography comprise a liquid medium having high insulating properties and low dielectric constants, such as petroleum aliphatic hydrocarbons, having dispersed therein organic or inorganic pigments or dyes, e.g., carbon black, nigrosine, phthalocyanine blue, etc., and natural or synthetic resins, e.g., alkyd resins, acrylic resins, rosine, synthetic rubbers, etc., and further contained therein a polarity regulator, such as metallic soaps, lecithin, linseed oil, higher fatty acids, polymers containing vinylpyrrolidone, etc. In these liquid developers, resins are dispersed as insoluble latex particles having a diameter of from 0.1 .mu.m to several hundreds .mu.m. However, since bonding between soluble resins for dispersion stability or polarity regulators and the insoluble latex particles is insufficient in the conventional developers, the soluble resins and regulators are apt to diffuse into the solution. As a result, the soluble resins for dispersion stability are separated from the insoluble latex particles during long-term preservation or repeated use to cause sedimentation, aggregation or accumulation of the particles or unclear polarity. The aggregated or accumulated particles are scarcely redispersed in the medium so that they adhere to various parts of a developing machine, which leads to stains on image areas or developing machine trouble, such as clogging of a feed pump, and the like.
In order to overcome these disadvantages, it has been proposed to chemically bind the soluble resins for dispersion stability and the insoluble latex particles as disclosed, e.g., in U.S. Pat. No. 3,990,980. The inventors experimentally confirmed that spontaneous sedimentation of particles can be suppressed to some extent by this technique, but such an improvement in dispersion stability is still insufficient in terms of redispersion stability for practical use. More specifically, when these liquid developers are practically used in an actual developing apparatus, toners are adhered to various parts of the apparatus to form a filmy solid, which is difficult to redisperse and ultimately causes the apparatus to malfunction, stains the image, and the like. Further, according to the process of this patent, it is very likely to produce particles of broad size distribution containing a large proportion of coarse particles or poly-dispersed particles having two or more mean particle sizes, and, in order to prepare mono-dispersed particles having a narrow particle size distribution, combinations of dispersion stabilizers used and monomers to be insolubilized are strictly limited. Furthermore, this process finds difficulty in obtaining mono-dispersed particles having a narrow size distribution and a desired mean particle size, only resulting in production of coarse particles of 1 .mu.m or greater or very fine particles of 0.1 .mu.m or even smaller. In addition, the dispersion stabilizers are prepared through complicated and time-consuming steps.
In order to eliminate the above-described problems, European Patent 155788A1 corresponding to Japanese patent application (OPI) No. 179751/85 (the term "OPI" as used herein means "unexamined published application") discloses that a monomer to be insolubilized is copolymerized with a monomer containing a long-chain alkyl moiety to produce insoluble resin particles, thereby improving dispersibility, redispersibility and preservation stability of insoluble particles.
On the other hand, printing of a large number of prints (i.e., more than 5000) by the use of a master plate for offset printing in an electrophotographic system has recently been attempted. In particular, improvements on master plates have made it possible to obtain more than 10,000 large size prints per plate. Further, efforts to shorten the operation time of the electrophotographic printing plate system by rapid processing of the development-fixation step have also been made.
According to the experiments conducted by the inventors, the particles prepared by the technique dislcosed in European Pat. No. 155788A1 were satisfactory in monodispersibility, redispersibility and preservation stability but did not satisfy performance requirements in view of durability on printing large-sized sheets or rapid fixing processing.